1. Field of the Invention
The invention relates to detecting entangled photons. In particular, the invention relates to discrimination between entangled photons and non-entangled photons.
2. Discussion of Background Information
Multiple photons may be random or entangled. Random photons are not entangled together and exist as independent entities. In contrast, entangled photons have a connection between their respective properties. Measuring properties of one or more photons in a set of multiply-entangled photons determines properties of the rest of the photons in the set. As understood by those of ordinary skill in the art and by way of non-limiting example, the quantum mechanical state of a set of n>2 multiply-entangled photons cannot be factored into a tensor product of n separate states.
Two photons entangled together are referred to as an entangled-photon pair (also, “biphotons”). Traditionally, photons comprising an entangled-photon pair are called “signal” and “idler” photons, where the signal photon has higher energy. Measuring properties of one photon of an entangled-photon pair determines properties of the other photon, even if the two entangled photons are separated by a distance. As understood by those of ordinary skill in the art and by way of non-limiting example, the quantum mechanical state of an entangled-photon pair cannot be factored into a tensor product of two individual quantum states.
Existing techniques for detecting entangled-photon pairs rely on single-photon counting to detect individual photons that form an entangled pair. These techniques use standard electronic devices to detect and correlate individual photons. Two photons detected in a short temporal interval may indicate that the photons form an entangled-photon pair. The entangled-pair count rate of existing techniques is limited by the speed of photon counting and correlating electronics. In particular, electronics overload limits the intensity of entangled beams that can be considered.